Exposure to high doses of arsenic-contaminated drinking water has been shown to have a significant adverse impact on prenatal growth and childhood cognitive development. Much less is known about the child health effects of the lower doses of arsenic exposure typical of U:S. populations despite evidence of adverse effects such as increased cancer risk at such levels in adults. Recent research suggests that diet may be the most significant source of exposure to arsenic for the general U.S. population, particularly children. Understanding the roles of diet and water as sources of arsenic is vital to assessing exposure risk especially in young children for whom developmental toxicant exposure can be highly deleterious. The New Hampshire Birth Cohort (NHBC) recruits from a population where about 15% of households have private wells with arsenic concentrations above the current maximum contaminant level of 10 pg/L. Thus, it provides a unique opportunity to evaluate early childhood arsenic exposure via drinking water and diet and to quantify physical growth and neurodevelopmental effects of such exposure. To characterize early childhood arsenic intake, we will combine consumption data from repeated dietary assessments with arsenic concentrations measured in each child's drinking water and foods. We will then explore how intake through water and diet are associated with urinary and toenail biomarkers of arsenic exposure. These arsenic biomarkers will then be evaluated for their association with altered growth (weight, length/height), and adiposity (body mass index, waist circumference, skinfold thicknesses) during the first five years of life, as well as impairment in behavioral skills and executive function at ages 3 and 5 years, and decrements in cognitive ability and motor proficiency at age 5 years, all neurodevelopment skills for which prior research supports likely arsenic sensitivity. The in utero and early childhood periods are windows of heightened vulnerability to environmental contaminants because of lower body mass and rapid development. Children also have specific dietary patterns that often include foods high in arsenic, especially rice and rice-based products. This innovative project will combine detailed longitudinal assessment of arsenic exposure and growth and neurological development to better understand critical children's health impacts of this common contaminant.